A semi-empirical model is developed to predict the extent of the reaction between calcined Greer limestone and hydrogen fluoride. Grain expansion due to the reaction, and the effect of changing porosity on the effective diffusivity of hydrogen fluoride through the calcined Greer limestone were considered in the model. Initial effective diffusivity of HF through the calcined Greer limestone and the experimentally obtained kinetic rate constants were utilized in the model. The model was validated against the data reported by Byer,et al. (1) and has shown to predict accurately the percent conversion of calcined Greer limestone below 790°C.